Latch circuit



SePtl, 1959 F. v. ADAMS LATCH CIRCUIT Filed oet. 18, 1956 INVENTOR.

FRANCIS v. ADAMS ATTORNEY SQmd United Stats LATCH CERCUT ApplicationOctober *13, '1956, Serial No. 616,765

2 Claims. (Cl. Z50- 27) The present invention relates to an improvedbistable device capable of `being set to a first or second stable state.

More particularly, the present invention relates to that class ofbistable devices in which a number of tubes are cascaded and a feedbackcircuit provided so as to form a closed circuit. Each tube is designedto assume a stable condition of maximum or minimum conductivity whichmay be referred to as an On and Off condition, respectively. Thecondition of each tube is determined by the condition of the precedingtube so that the condition of one tube will determine the conditions ofall tubes. Once set in a given condition, each tube will remain in thatcondition until some external stimulus is applied. An example of thistype circuit is shown in the patent to E. S, Hughes, Jr., Patent No.2,628,309.

In prior devices, such as shown in Hughes, the circuit comprises threetubes, two inverters and a cathode follower. The plate of the firstinverter is connected to the grid of the second inverter, the plate ofthe second inverter is connected to the grid of the cathode follower andthe cathode of the cathode follower is connected to an output circuit inaddition to providing a connection back tothe grid of the firstinverter. In circuits of this type, where the condition of one tube willdetermine the conditions of all tubes because of the closed circuitarrangement, it is yapparent that the speed of operation will dependupon the time it takes for all tubes to reach a stable condition. Wherethe output of such a device is loadedA with a large amount ofcapacitance, the time it takes for the cathode follower to dischargethis capacitance will limit the speed of response of the device sincethe first inverter will vdepend for operation upon the cathode voltageof the cathode follower.

If the device is one of a plurality in a ring circuit driven by a sourceof ring drive pulses, failure to assume an On position before the nextdrive pulse will cause failure of the ring. This is caused by the factthat succeeding devices are operated by a pulse from a preceding devicewhen the preceding device goes from an On to an `Oii position. If thepreceding device does not assume an On position, here will be no pulseto operate the succeeding device. An On position is defined `as asituation in which the cathode follower conducts heavily and 4an Offposition is defined as a situation in which the cathode follower is onlyslightly conducting.

Since ring circuits in computer applications characteristically workover long lines, there is a large amount of capacitance associated withthe output of each device in the circuit.

It is therefore an object `of this invention to provide an improvedbistable device.

It is a further object of this invention to provide a bistable devicerelatively insensitive to load variations.

It is lanother object of this invention to provide a bistable devicewhich operates at a higher frequency than those previously known.

Other objects of the invention will be pointed out in arent O "ice thefollowing description and claims and illustrated in the accompanyingdrawing, which discloses, by way of an example, the principle of theinvention and the best mode, which has been contemplated, of applyingthat principle.

In the drawing:

The circuit shown in the figure consists of thee tubes V1, V2 and V3. V1`and V2 function as inverters and V3 functions as a cathode follower.The normal condition of this circuit, which shall be referred to as anOff condition, is where tube V1 is nonconducting, tube V2 is conductingand tube V3 is only slightly conducting. The output taken from thecathode of V3 at this time will be at some negative potential level. TheOn condition of this circuit is where V1 is conducting, V2 isnonconducting and V3 is conducting heavily. The output at this time willbe 'at some positive potential level.

The circuit is shown as it would be connected into a ring circuit withan output to the next stage taken from tubes V2 and V3. This outputwould be connected to a terminal in a subsequent stage labeled Set asshown in the instant stage.

A Reset for manually switching from an On to an Off condition is shownconnected to the input of tube V1. An advance pulse input is shown whichwould be connected to a ring drive for switching the stage from On toOff where the circuit is used as a stage of a ring.

With tube V1 nonconducting, the plate of said tube assumes the potentialat point i8. The plate of tube V1 is connected to the grid of tube V2.The positive potential at the plate of V1 is suiicient to overcome thenegative bias applied to V2 and V2 conducts.

The positive voltage at 9 is connected through va large resistor itl,through diode 13 to the cathode of V3, through resistor 4l to the plateof V2. With V2 con ducting, current will ilow from 9 through V2. Withcurrent flow through resistor 4l, the grid of tube V3 will be at a morenegative potential than the cathode of V3 and V3 will therefore bebiased near cutoff. Resistor 10 is very large compared to the resistor4l and the resistor 35 in the cathode circuit of V2 so thatsubstantially the entire voltage drop between voltage source 9 and thecathode voltage V2 appears across resistor 10. This voltage drop bringsthe point 8 below ground potential and keeps V1 cut oif so that thecircuit will be stable in the OI position. With V2 conducting and V3only slightly conducting, the output will be at a negative potential.The diode d4 prevents the line from going too far negative.

To operate the circuit to an On condition, a negative pulse is appliedat terminal 2o labeled Setf This pulse is connected to the grid of tubeV2 through a voltage dividing network composed of resistors 2l, 23 andcapacitors 2li, 22. This negative pulse drives the grid of tube V2 belowcutoff and the tube V2 is operated to its nonconducting condition. Withtube V2 nonconducting, the potential at point 3S will be the same asthat for the cathode of tube V3 (no current iiow through resistor 41)and tube V3 will conduct heavily. With tube V3 conducting, the cathodeof V3 will rise to a potential high enough to prevent too large avoltage drop across resistor 10 and the tube V1 will be biased toconduction. With tube V1 conducting, the plate of tube V1 isapproximately at ground potential. V2 was cut olf by the pulse onterminal 2o and, since V2 is normally biased to cutoff by source 32, thepresence of ground potential on the plate of tube V1 is not sufcient tomake tube V2 conduct. The circuit is now in a stable On condition andwill remain in this condition until it is reset to an Off condition by anegative pulse on the reset or advance pulse line.

v resistance.

AThe output of this circuit is taken from a parallel combination of theoutputs of tubes V2 and V3. From the figure it can be seen that theplate of tube V2 is connected through a -resistor 4l to the outputterminal and the cathode of tube V3 is connected directly to the outputterminal. Any output pulse therefore will be a direct function of theoutputs of tubes V2 and V3.

At this point it may be well to digress for a moment and consider thebehavior of a cathode follower when driving a load. When a cathodefollower conducts, current will iiow from the plate to the cathode andthen to the load. From this it can be seen that a cathode follower willdeliver a large amount of power (current) at the positive excursion ofan output pulse. When the cathode follower is biased near cutoff,current ow is from the line into the cathode circuit of the cathodefollower. When the output contains considerable capacitance, as it doeswhere these devices are used, the capacitance must discharge through thecathode resistor and the load resistance in parallel (shown in brokenlines in the figure). in an R-C circuit, lthe voltage decaysexponentially in accordance with the circuit constants. One wayto helpthis condition is to greatly lower the value of the cathode resistor butthis necessitates either a larger tube with a higher plate dissipationor tubes in parallel.

Since the cathode volta-ge will decay exponentially in accordance withthe capacity of the line and the cathode resistor, it will take a givenamount of time before the voltage reaches'some predetermined level. Ifthe instant device is used in a ring circuit driven at somepredetermined frequency, it can be seen that a failure of one stage tochange condition before the neXt advance pulse arrives will cause thering to fail.

A plate driver, when connected to an output, functions when conductingto accept current from the output line. When nonconducting, the platedriver is eifectively an open circuit and current to the line would befrom the plate supply. Therefore, for a negative excursion of an outputpulse, i.e., plate driver conducting, any capacitance of theline wouldbe discharged through the plate driver, while for a positive excursion,i.e., plate driver nonconducting, current would be from the plate supplyand the plate driver would have the same disadvantages as the cathodefollower' would for a negative excursion.

Y To make the device of the figure more independent of load variationswith regard to changes in condition of the device, a plate driver and acathode follower have been combined to furnish an output pulse whichwill tum the device On or Gif in a faster time than previously known.

As previously stated, a plate driver such as V2 will have optimumoperation when generating the negative excursion of an output pulsetaken across the plate since the circuit is through the tube V2 to thecathode circuit where the resistor 35 has a small. resistance. A cathodefollower such as V3 will have optimum operation when gen# erating thepositive excursion of an output pulse taken across the cathode since thecircuit is through the tube V2 from the plate supply where the resistor40 has a small Equally important is the fact that a cathode followerwill present a low irnpedence to the line between pulses and minimizespurious Crosstalk Therefore, in using a plate drive such as V2 and acathode follower such as V3 in combination, an optimum output waveformis achieved.

Y`From the foregoing analysis it can be seen that the tubes of theoutput circuit determine the sharpness of the output pulse. Since thetime which it takes for the entire circuit to assume a stable state isdependent upon the time it takes for the output pulse to assume aparticular voltage level, it can be seen that the frequency at which thecircuit will respond is directly dependent upon the output waveform.With the circuit as shown,

the improvement in output waveform has materially increased the responsetime.

To further improve the response time of the present circuit, a capacitor14 has been inserted in the feedback circuit. The capacitor allows thegrid of tube V1 to follow the feedback voltage from the output morequickly than by merely using the diode.

To prevent the negativeegoing pulse at terminal Z6 from driving the gridof V2 below cutoff and consequently increasing the time that it takesfor the grid to approach the normal voltage for cutoi, a connection wasmade kto the cathode of V2 to the point Z5 of the voltage dividingnetwork. A diode 29 isolates the network from the cathode potential attimes when there is no'input pulse at 26. For a large negative pulse,the diode conducts and keeps the point 25 at a predetermined level belowthe cathode voltage dependent upon the resistor 35.

The diodes 33 and 44 are used to prevent the output waveform from goingtoo high or too low, vi.e., for clipping. 2

The diodes 11 and 12 form an Or circuit wherein terminals 4 and 5 aremaintained at a potential high enough to prevent current flow fromsource 9 through diodes 11 and 12. When it is desired to reset oradvance the circuit from an On to an Off condition, a negative pulse issupplied. The resistor 10 will then permit the voltage at the grid oftube V1 to drop below cutoiitl and the circuit will be turned Cif.

While there have been shown and described and pointed out thefundamental novel features of the invention as applied to a preferredembodiment, it will be understood that various omissions andsubstitutions and changes in l the form and details of the deviceillustrated and in its operation may be made by those skilled in theart, withf out departing from the spirit of the invention. It is theintention, therefore, to be limited only as indicated by the scope ofthe following claims.

What is claimed is:

1. A bistable device comprising a first, second and third electron tube,each tube containing -at least a plate, grid and cathode, a iirst andsecond series connected parallel resistance capacitance sectionconnecting the plateof said tirst tube to the grid of said second tube,

' means connecting the cathode of said second tube to a junction betweensaid first and second series connected sections whereby the voltage atsaid junction will be a function of the voltage of said cathode, meansconnecting the plate of the second tube to the grid of said third tube,means connecting the cathode of said third tube to the ,grid of saidfirst tube whereby the conduction of any onetube will determine theconduction of all tubes to set -saiddevice in either a rst or secondstable state,

an input connected to the junction between said Erst References Cited inthe le of this patent UNITED STATES PATENTS 2,307,308 Sorensen Jan. 5,1943 2,434,916 Everett Ian. 27, 1948 2,547,213 Johnson et al Apr. 3,1951 2,586,409 White Feb. 19, 1952 2,628,309 Hughes Feb. 10, 1953y2,719,227 Gordon Sept. 27, 1955 2,790,076 Mason Apr. 23, 1957

